PROJECT SUMMARY This project seeks to use real-time geospatial methods to investigate relationships between Global Positioning System (GPS)-defined activity space neighborhoods, social and sexual networks, and HIV pre-exposure prophylaxis (PrEP) uptake and adherence cross-sectionally and longitudinally among young Black men who have sex with men (MSM) in the Chicago IL metropolitan statistical area (MSA) followed over two years. We will enroll 350 Black MSM in the proposed NIH-funded uConnect Neighborhoods and Networks Cohort Study to address the aims of the research. Eligibility requirements include: HIV-seronegative; self-report no restrictions to usual physical activity; no plans to move outside of Chicago IL MSA in the next two years; and willingness to complete a two-week GPS protocol at nine time-points over two years. Participants will wear the GPS device following protocols we have used in our previous feasibility research projects, including studies among Black MSM. In this longitudinal study, three months after completing the initial 2-week GPS protocol, participants will carry the GPS device for an additional 2-weeks every three-months over the two-year study period?for a total of nine times. GPS activity space environment and social/sexual networks data at baseline could potentially influence PrEP outcomes over time. Also, multiple GPS measures and social/sexual networks (at different time points) can better capture the breadth of people?s exposure to neighborhood-level factors and dynamics in social/sexual networks. The proposed study will be the largest GPS study of HIV disparities in any MSM population and presents a remarkable opportunity to study environmental influences on HIV prevention. Findings from the proposed research will impact HIV prevention intervention activities. First, it will inform specific neighborhood-level policy interventions. For example, increased community efforts to combat lesbian, gay, bisexual and transgender hate crime neighborhood rates through increased local police attention in high- crime locations may be an HIV prevention intervention that can reduce HIV health disparities. Second, from the GPS dataset we will know the travel patterns of Black MSM and therefore be able to identify optimal geographic locations for HIV prevention interventions. This will advance the literature given that such interventions are seldom geographically targeted. Third, this research will facilitate the identification of geographic locations suitable for recruiting Black MSM in research studies, an improved method of venue- based sampling, in this understudied population. Fourth, examining changes in spatial mobility (i.e. activity spaces) over time will be useful in knowing whether the risks of particular spaces changes or remain constant because different neighborhoods will have different risk profiles. Changing risk environments would suggest that prevention requires significant dynamism and fluidity, as well as rapid change-detection feedback loops. Finally, our dynamic network analysis we propose will deepen understanding of the effects of social and sexual networks on HIV prevention behaviors and will improve network-based HIV prevention interventions.